#define _CRT_SECURE_NO_WARNINGS 1

class Solution {
public:
    void duplicateZeros(vector<int>& arr) {
        int cur = 0, dest = -1, n = arr.size();
        while (cur < n) {
            if (arr[cur])
                dest++;
            else
                dest += 2;
            if (dest >= n - 1)
                break;
            cur++;
        }
        if (dest > n - 1) {
            arr[n - 1] = 0;
            cur--;
            dest -= 2;
        }
        while (cur >= 0) {
            if (arr[cur])
                arr[dest--] = arr[cur--];
            else {
                arr[dest--] = 0;
                arr[dest--] = 0;
                cur--;
            }
        }
    }
};